<template>
  <div class="One">
    <div>One---oneCount</div>
    <button @click="$root.a++">add</button>
  </div>
</template>
<script>
export default {
  name: 'One',
  mounted() {
    console.log(this.$root);
  },
};
</script>
<style>
.One {
  border: 3px solid #000;
}
</style>
